The abundance cycle and influence factors on red tide phenomena of Noctiluca scintillans (Dinophyceae) in Dapeng Bay, the South China Sea

?he seasonal occurrence of Noctiluca scintillans and the causative mechanisms of red tide phenomena were investigated in the northwestern part of Dapeng Bay, the South China Sea, from March 1990 to June 1992. It occurred in the plankton usually from January through May/June with peak abundance from March to early May. At least eight red tides of this organism, when the surface water was covered with the pink patches and slicks, were observed during the investigation period, and all of them took place between March and early May. Noctiluca occurred at temperatures from 15.8 to 28.6 degrees C, and the population density was the highest at temperatures between 19 and 25 degrees C. It disappeared from the plankton in summer and fall when the temperature warmed to lethal level of similar to 30 degrees C and then the population probably was transported to outside of the bay. During the peak period of abundance between March and May, the log transformed average density of Noctiluca at each station showed a significantly positive relationship with average water temperature, indicating that within its optimum temperature a higher temperature promotes the growth of Noctiluca population. Sharp declines of the population were associated with abrupt decreases in salinity due to heavy rain. Hence, rich food supply is basically necessary for Noctiluca to reproduce massively, but the suitable temperature, stable muggy weather without heavy rain are considered to be the important factors for the Noctiluca blooms. The surface convergence, due to the geographical and tidal features, may also constitute a direct driving force leading to its frequent occurrences in the study area. The Noctiluca density showed a negative relationship with chlorophyll a concentration, indicating that this species gives a predation pressure on the phytoplankton.